Electron therapy
Class 1: fundamentals
Laurence Court
lecourt@mdanderson.org
Reference: Faiz M. Khan, The Physics of Radiation Therapy
Slide acknowledgements: Karl Prado, Rebecca Howell, Kent Gifford,
Rajat Kudchadker, Ken Hogstrom and Khan’s book
1
Electron therapy
1.
2.
3.
4.
Fundamentals
MU calculations
Dose calculations
Special procedures
2
Characteristics of clinical electron beams
Surface
Dose
Depth of
90% Dose
Depth of
80% Dose
X-Ray
Contamination
3
Why treat with electrons?
• Region of fairly uniform dose, then rapid falloff
• Treat superficial targets
• Avoid dose to deep and/or adjacent tissues
4
Clinical context: Why treat with electrons?
5
Head and neck
• Fields for
postoperative
irradiation of a patient
with advanced cancer
of the laryngopharynx
• Electrons used for offcord reduction
Image from: Amdur RJ et al. Postoperative irradiation for squamous cell carcinoma of the head and neck: an
analysis of treatment results and complications. Int J Radiat Oncol Biol Phys 1989;16:25–36.
6
Head & Neck
Posterior Strips (9 & 12 MeV)
7
Insert Positioning
8
Parotid
• 46-year-old lady with T2, N0
mucoepidermoid carcinoma of the left
parotid gland.
9
No Bolus
10
Non-Beveled Bolus
11
Beveled Bolus
(16 MeV)
12
Pediatric CNS
• 3-year-old boy presented in fall of last year
with tremors and found to have a large mass
in the right parietal lobe with craniotomy.
Biopsy confirming choroid plexus
carcinoma. There is evidence of
leptomeningeal disease both intracranially
as well as in the spine.
13
Pediatric CNS
(12 MeV)
14
Transverse CNS
15
Bi-Lateral Chestwall
• 70-year-old, postmenopausal female that had
a left modified radical mastectomy in 07/01,
and right modified radical mastectomy in
06/02 along with adjuvant chemo and
radiotherapy.
16
Bi-Lat ChestWall Electron
(5 & 7 MeV )
17
Bi-Lat Chestwall Skin Rendering
18
Intact Breast
• 54 years old and has a T1, N0 invasive
carcinoma of the right breast which is ductal
in origin, nuclear grade III. She underwent
segmental mastectomy with negative
sentinel node evaluation.
19
Breast Boost
(20 MeV)
20
Mesophelioma
• Extrapleural pneumonectomy (EPP) followed by RT
• Heart, liver, lung, kidney, cord, esophagus
21
Mesothelioma
• 180cGy to 5400cGy
• Abdomen block to shield
stomach and liver
• Start heart block at
1980cGy
• Block cord at 4140
22
Why treat with electrons?
• Region of fairly uniform dose, then rapid falloff
• Treat superficial targets
• Avoid dose to deep and/or adjacent tissues
23
LINAC in e- Mode
Khan,
Figure 4.8B
• Scattering Foil
• Monitor ionization chambers
• Field defining Light
• Collimator System
– 2o e- collimation = cone,
downstream of photon
collimatorsplaced close to
skin to dec. e- scatter in air
– 3o e- collimation = cutout
downstream of
conecustom shape field to
match Rx site
24
Electron scattering system
25
Electron Cones
• Always use a electron cone.
26
Electron Cones
• Available cone sizes
for the Varian 21EX:
20x20
– 6x6, 10x10, 15x15, 20x20,
25x25
15x15
10x10
6x6
25x25 (18lbs)
27
Electron Cutout
28
Electron Cutout
• Further field shaping
with cut-out.
29
Cone Interlock
30
Energy loss
Two Types of Electron Interactions
1. Collisional Interactions
(ionization and excitation):
Incident electron interacts
with atomic electrons in the
absorbing medium
e-
2. Radiative Interactions:
Incident electron
interacts with atomic
nuclei in the absorbing
medium.
e31
Electron Stopping Power

Stopping Power - Energy lost per unit path
length in the medium  MeV/cm


Stopping power depends on the density of the
absorbing medium. Thus……
Mass Stopping Power = Stopping power divided
by density MeVcm2/g
 MeV 


3
2


MeV
cm
MeVcm
cm




 


g
 g   cm  g 
 3
 cm 
32
Stopping Power
• Two Components:
1.Collisional Stopping Power
2.Radiative Stopping Power
S
S
S
 
     
  Total    Col    Rad
33
Khan, Figure 14.1
Collision Stopping Power:
 Tapers off with increasing energy
 Higher for Lower Z
 ~2MeV/cm for water for E>1MeV
Radiative Stopping Power:
 Large increase with
increasing energy
 Higher for higher Z
34
(Khan)
Restricted Collisional Mass Stopping Power
• Energy lost by a charged particle in a medium is not always equal to
the energy absorbed in a target, especially when the target is small
with respect to the range of secondary electrons produced, some of the
secondary electrons (-Rays) do not deposit dose locally.
• Restricted Collisional Mass Stopping Power  The energy (E) lost by an
electron per unit path length (l) as a result of collision interactions with
atomic electrons in which the energy loss is less than .
•Basically it is the collisional stopping power without the high energy
-rays.
ICRU
Nomenclature
L
 dE 
 

 
   col ,  d  col ,
35
Absorbed Dose
Restricted collision mass stopping power
36
Electron Scattering
• Multiple-coulomb scattering interactions
between incident electrons and (mostly)
nuclei of the medium
• Scattering power  Z2E-2
From AAPM Summer School 1991
37
Effect of scattering on depth-dose curves
(3) + angle
(2) + scatter
(2) With
scatter and
angle
(3) Experimental
38
Characteristics of clinical electron beams
Surface
Dose
Depth of
90% Dose
Depth of
80% Dose
X-Ray
Contamination
39
Some PDD descriptors
R50 = the depth
(cm) at which
the dose is 50%
of the maximum
dose.
Rp = the depth (cm) of the
point where the tangent to
the descending portion of the
curve intersects the
extrapolated background
X-Ray
Contamination
40
Energy specification
41
Mean Energy, E0
• The MEAN ENERGY of an electron beam, at
the phantom surface is given by:
E0  C4  R50
 C4 as defined by TG21 is 2.33 MeV/cm
 C4 according to more recent Monte-Carlo
calculations is 2.4 MeV/cm
42
Most probable energy
According to the Nordic Association of
Medical Physics:
• C1=0.22 MeV
• C2=1.98 MeV cm-1
• C3=0.0025 MeV cm-2
• (in practice, OK to ignore beam divergence)
43
Energy at Depth, (Ep)z


z

E z  E0  1 
 R 
p 

Where:
z is depth in tissue
Rp is the practical range
(same equation for (EP)Z
EZ
E0
Z
44
Characteristics of Clinical Electron Beams
• Depth of the 80% Dose:
– Equal to approximately Enom/2.8 :
E n o m in a l
E n om / 2 .8
A c tu a l
6
9
12
16
20
2 .1 4
3 .2 1
4 .2 8
5 .7 1
7 .1 4
2 .2 0
3 .3 0
4 .3 0
5 .5 0
7 .0 0
MDACC
21EX
– Depth of 90% is approximately Enom/3.2
E n o m in a l
E n om / 3 .2
A c tu a l
6
9
12
16
20
1 .8 8
2 .8 1
3 .7 5
5 .0 0
6 .2 5
2 .0 0
3 .0 0
4 .0 0
5 .0 0
6 .1 0
45
Characteristics of clinical electron beams
• Depth of maximum dose (R100):
• For E<12MeV, R100 equal to
approximately E/4
• Remember PDD very flat for higher
energy beams
• Practical Range:
– Equal to approximately 1/2 nominal energy:
E
n o m in a l
6
9
12
16
20
E
nom
3 .0
4 .5
6 .0
8 .0
1 0 .0
– Energy loss is about 2 MeV / cm
/ 2
R
p
3 .1 5
4 .5 8
6 .0 4
7 .6 6
1 0 .1 3
46
MDACC 21EX
Electron Depth Dose Curves:
The approximate 4,3,2 rule of thumb
R100
E0

4
R8090
E0

3
4
3
E0
RP 
2
90
80
R100
R90 R80
2
Rp
47
Entrance dose
• Surface dose increases with
increasing energy
– At lower energies electrons are
more easily scattered through
larger angles.
Enom
Surface dose
(%)
6
72
9
78
12
83
15
87
18
91
48
X-ray contamination
• Increases with energy
• Varies with accelerator design
• Defined at RP+2 cm
Enom
X-ray %
6
9
12
16
20
0.7%
1.2%
1.9%
3.7%
5.9%
49
Khan, figure 14.1
50
Raphex Question: T64-67, 2003
•
Match the electron energy with the feature described below:
A. 6MeV
B. 9MeV
T64. Has Highest Surface Dose.
T65. Has Range of 6cm in tissue.
T66. Has 90%dd at 2.7cm.
C. 12MeV
D. 16MeV
E. 20MeV
T67. Has sharpest falloff between 80%
and 20%.
Bonus: Has highest dose beyond the
practical range
51
Raphex Question: T54, 1999
•
An electron beam of how many MeV would be
most suitable to treat a volume extending to a
depth of 3cm of area 10x10cm?
A.
B.
C.
D.
E.
3
6
10
15
20
52
Raphex Question: T46, 2001
•
Compared with 6MeV electrons, 16MeV electrons
have:
a) A greater surface dose.
b) A lower bremsstrahlung tail.
c) A sharper fall-off between 80% and 20%
isodose levels
53
Raphex Question: T58, 2002
• Which of the following is true regarding electron
beams? The surface dose:
A. Is about the same as that of a photon beam with the
same energy
B. Is lower for a beam with a scattering foil than for a
scanned beam.
C. Is about the same as that of a superficial x-ray
beam.
D. Increases as energy increases
54
Raphex Question: T68, 2003
•
The dose beyond the practical range is
primarily due to:
A.
B.
C.
D.
Very low energy electrons.
The highest energy electrons in the spectrum.
Characteristic x-rays generated in tissue.
Bremsstrahlung.
55
What energies do you think we might use?
(a)
(b)
Choice: 6, 9, 12, 15, 18MeV
Bolus or no bolus?
56
Electron Isodose Curves
• Isodose Curves – Scattering of electrons very
important factor in SHAPE of isodose curve.
• As the beam penetrates the medium, the beam
expands rapidly below the surface due to
scattering!
• Spread of isodose curve depends on:
1.
2.
3.
4.
Isodose Level
Energy
Field Size
Collimation
57
Electron Isodose Curves
High Energy Electron Beams
Low Energy Electron Beams
• LOW isodose levels bulge out
• ALL isodose levels bulge
• HIGH isodose levels show lateral
out!
constriction, which becomes worse
with decreasing field size
7 MeV
18 MeV
58
External electron shielding
• Thickness of shielding should give < 5% transmission.
– If weight or thickness no problem, can use more than
minimum required.
• When placing shields directly on patient use minimum
thickness to achieve desired reduction in dose.
– Eye shields
– Internal shields
• Rule of Thumb: Minimum thickness of Pb for blocking
electrons in mm is given by electron energy INCIDENT in
the lead divided by 2.
Eo MeV
mmPb 
2
59
Internal Shielding
• Internal shielding can be useful to protect
structures beyond target volume.
• Commonly used for:
– Buccal mucosa, lip, eye lid lesions.
• Electron backscatter from the lead can enhance
dose near the shield.
– Magnitude of the Increase: 30-70% in the range of 1 to 20
MeV, having higher value for lower energies.
60
Internal Shielding
• Electron back scatter Factor (EBF) – The
quotient of dose at interface with lead in place
to that with homogeneous polystyrene
phantom at the same point
EBF  1  0.735e
( 0.052 E z )
where,


z

E z  E0  1 
 R 
p 

61
Internal Shielding
• To dissipate the effect
of elect of electron
backscatter from Pb
shield, place suitable
amount of low Z
absorber between lead
shield and preceding
tissue interface.
• Typically want to
reduce transmission of
the backscattered
electron intensity to
≤10%.

Thickness of Low Z (=1) material required
to absorb the backscattered electrons is
determined using the data in the figure
below:
Khan, Figure 14.40
62
Internal Shielding Example:
• A buccal mucosa lesion is treated with a 9MeV
electron beam incident externally on the cheek.
• Assuming the thickness of the lesion is 2cm,
calculate:
a) The thickness of lead required to shield the oral
structures beyond the cheek.
b) Magnitude of electron backscatter.
c) Thickness of bolus or Al to absorb the backscattered
electrons.
63
Internal Shielding Example:
a. The thickness of lead required to shield the oral structures
beyond the cheek.


Incident Energy = 9MeV
Need to calculate energy at depth = 2cm

 
z
2 


E z  E0 1 
 91 
 5MeV

 R   4.5 
p 

 Recall: Rule of Thumb: Minimum thickness of Pb for
blocking electrons in mm is given by electron energy
INCIDENT in the lead divided by 2.
5
mmPb   2.5
2
64
Internal Shielding Example:
b. Magnitude of electron backscatter.
EBF  1  0.735e
( 0.052 E z )
EBF  1  0.735e
( 0.052 5 MeV )
 1.57
The lead shield causes a 57% increase in
dose at the tissue interface.
65
Internal Shielding Example:
c. Thickness of bolus or Al to absorb the backscattered
electrons.
 From Figure 14.40, a
5MeV beam 10mm of
polystyrene or bolus
required to reduce the
transmission of
backscattered electrons
to 10% transmission.
 Using equation
AltAl=bolustbolus, the
equivalent amount of
Al is 4mm
(AL=2.7g/cm3)
Khan, Figure 14.40
66
Raphex Question: T70, 2003
•
The thickness of lead required to shield a 6MeV
electron beam is approximately___mm.
A.
B.
C.
D.
E.
0.5
3
6
12
24
67
Problems of Adjacent Electron Fields
• Adjacent Electron Fields
– Abutting at surface – Hot spots in junction region.
– Separated at surface – Cold spots in junction region.
• Electrons are typically used to treat lesions close
to skin surface.
– Hot spot may be more acceptable than cold spot.
68
Problems of Adjacent
Electron Fields
• Adjacent electron
Fields with different
gaps at surface.
• As gap decreases
from 1.5cm to 0.5cm,
cold spot is reduced,
but hot spot is
increased.
Khan, Figure 14.30
69
Problems of Adjacent Electron/Photon Fields
• Adjacent Electron/Photon Fields
– Hot spot on Photon side
– Cold spot on electron side
Due to outscattering of
electrons from electron field.
• The extent of the hot and cold spots depends on the
electron beam SSD
– When SSD is increased:
• Electron profile becomes less flat as a result of increased
electron scattering by air.
• Hot and Cold spots spread out over larger areas without
large change in magnitude.
70
Adjacent Electron/Photon Fields
• Adjacent
Electron/photon fields.
A. SSD = 100
B. SSD = 120
Hot/cold spot covers
larger area when
electron field at
extended SSD.
71
Raphex Question: T59, 2002
•
Regarding electron field junctions, which of the
following is true? When the light fields of two adjacent
electron fields are matched on the skin:
A. The dose variation across the junction will be within 5% if
the electrons have the same energy and light fields abut.
B. There will always be hot and cold spots because of the
shape of electron penumbra.
C. The formula: gap=(depth/sad)x(C1+C2)/2 gives the best
match.
D. Overlapping the fields by 0.5 cm generally gives the best
match.
72
Effect of Oblique Incidence
on Dose Distribution
• The broad electron  When a beam is obliquely incident on
beam can be
the patient’s surface:
represented as a large – Points at shallow depths receive greater
number of pencil
side scatter from adjacent pencil beams,
beams placed
which have traversed a greater amount of
the material.
adjacent to each
other.
– Points at greater depths receive less scatter.
73
Effect of Oblique Incidence
on Dose Distribution
• Increased dose at shallow depths
• Decreased dose at deeper depths
Expected
consequence of
oblique incidence
• In Reality as obliquity increases, the air gap between the
skin surface and the cone end increases.
• The depth dose at a point in an obliquely incident beam
is effected by both “pencil scatter effect” and “beam
divergence”.
74
75
Khan14.23
Hot
Spot
Hot
Spot
Hot
Spot
Hot
Spot
Hot
Spot
Hot
Spot
Surface Irregularities
76
Raphex Question: T59, 1999
•
An electron enters a patient’s surface obliquely. If
the MU are calculated for normal incidence, ALL
of the following can be expected EXCEPT:
A.
B.
C.
D.
Surface dose increases.
Depth of dmax decreases.
Depth of 90% isodose decreases.
Depth of 50% isodose increases.
77
Raphex Question: T61, 1999
•
When treating with high energy electron beams, one of
the problems with using bolus over part of the field is:
A.
B.
C.
D.
Calculating the thickness necessary.
Finding an appropriate material.
Dose inhomogeneity at the edge of the bolus.
The production of Bremsstrahlung.
78
Output – effect of field size
79
PDD- effect of field size
80
How small can I make a field?
• If a field produced by a lead cutout is smaller than the
minimum size required for maximum lateral dose build-up,
dose in open portion is reduced.
• The ICRU suggested Rp as the lower limit for field diameter.
– Lateral scatter equilibrium is achieved if field size created by the
cutout is > Rp (E/2).
• Khan showed lateral scatter equilabrium is achieved if
Req>0.88Ep,0
Should
be > E/2
(will talk about square81vs circle later)
Raphex Question: T60, 2002
•
Electron output (cGy/MU) depends on all
of the following EXCEPT:
A.
B.
C.
D.
Cone or applicator size.
SSD.
Size of custom insert.
Dose Rate at dmax
82
Raphex Question: T69, 2003
•
When a custom electron insert has dimensions
smaller than the range of the electrons, all of
the following are likely to occur except:
A.
B.
C.
D.
The output (cGy/MU) will be reduced
Surface dose will decrease.
PDD will decrease beyond dmax.
Dmax will shift toward a shallower depth.
83
Characteristics of clinical electron beams
Depth of
90% Dose
Surface
Dose
Depth of
80% Dose
Thank you!
X-Ray
Contamination
84